Paraphrasing Davis in the 1842 edition of the Manual
of Magnetism, pp 106-107:

In this instrument there are two semicircular electro-magnets
of the same size, both of which have freedom of motion. The lower semicircle
is supported by a pivot entering the upright pillar below it; its own axis
is hollowed to receive the pivot on which the upper semicircle revolves.
The two white disks are commutators furnishing alternating connections
to the two electromagnets.

When the apparatus is connected to a battery, the two coils
will spin in opposite directions.

This instrument, which cost $6.00 when new, is in the Collection
of Historical Scientific Instruments at Harvard University.